Amanda, I do know that in order to take your child into Mexico, you must have a notorized letter from the other parent stating it's ok. This is for land entry. For a cruise, I don't know, but when I lived in AZ people were all the time scrabling to get the letters when they were going to ROcky Point (the closest beach to AZ). If you do a search on ROcky Point or Puerto Penasco, you should be able to find FAQs that talk about it. Since it is only a letter from the other parent, and it only needs to be notorized, you should be able to get one pretty easily, even if you don't need it. Better to have it than not.
Back from the ped, now onto the PMs for Lanyards!